Your question is a bit open ended... Do you want to block everything but SMTP from a specific country? Or block all traffic to anywhere, except for SMTP from a specific country? Generally you are going to want to try and build rules with specific allows, followed by global drops for anything else.
It of course depends on you other rules, but something like this would allow SNMP inbound, but block all other traffic:
If you also want to block requests outbound to that country:
3. Name="Block all traffic outbound to El Salvador", SrcZone=LAN, SrcAddr=any, DstZone=WAN, DstAddr=Regions:SV, Application=any, Service=any Action=Drop
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